1. Field of the Invention
The present invention relates to a method for etching a silicon layer which makes it possible to provide a configuration having inclined sides through etching.
2. Description of the Related Art
Structures of devices such as flash memories having a floating gate must have a laminated layer configuration as shown in FIG. 1 before a polycide film to serve as a transistor gate is etched. That is, there is a WSi layer 31 which is the uppermost layer followed by a second polysilicon layer 30, an ONO layer 29 having a three-layer structure formed by an oxide film, a nitride film and an oxide film layer, a first polysilicon layer 28, an oxide film layer 27 and a silicon substrate 26. In this structure, the first polysilicon layer 27 to serve as a floating gate is etched in advance vertically as viewed in the cross section. A field oxide film 32 is locally formed in a location where the first polysilicon layer 28 is etched. As shown in FIG. 1, the sides of the hole formed by etching the first polysilicon layer 28 are inclined such that the hole is wider in the upper part. Such a laminated layer structure is etched using an etching apparatus for polysilicon.
While the two upper layers, i.e., the ONO layer 29 and second polysilicon layer 30, are etched without any problem under normal etching conditions, the oxide films in the ONO layer 29 can not be etched at a high etching rate as in a normal etching apparatus exclusively used for oxide films because a polysilicon etching apparatus is used. It is therefore preferable that the ONO layer 29 has a configuration that can be easily etched.
However, as shown in FIG. 2, when the first polysilicon layer 28 is etched in a configuration having vertical sides, the ONO layer 29 thereon will be also formed vertically. If the oxide film layers of the ONO layer 29 are etched vertically relative to the substrate, the thickness of the oxide film layers which is the effective thickness to be etched is equal to the thickness of the first polysilicon film 28. This consequently makes it very difficult to remove the ONO layer 29 and an ONO layer 33 can remain as shown in FIG. 3.
Therefore, in order to make the ONO layer 29 easy to etch, the first polysilicon layer 28 must have a tapered configuration is inclined to be wider in the upper part thereof instead of a vertical configuration.
However, conventional methods for etching have not been effective in providing a configuration having inclined sides as described above after etching. For example, Japanese patent Application Laid-Open No. 8-274078 discloses a method of preventing a gate electrode from being tapered by performing etching having two stages to form a side protection film with different degrees of ease. However, this method does not provide a tapered etching configuration.
It is an object of the invention to provide a method for etching a silicon layer wherein an etching configuration having inclined sides can be obtained.
According to a first aspect of the invention, there is provided a method for etching a silicon layer comprises the steps of forming a mask pattern on a silicon layer, etching the silicon layer halfway using a CF type gas or a mixed gas including the same and etching any residue of the silicon layer using at least one kind of gas selected from among a group of gases consisting of Cl2, HBr, SF6 and O2.
In this method for etching, the CF type gas may be at least one kind of gas selected from among a group of gases consisting of CF4, CHF3, CH2F2 and C4F8 and an oxide film having a thin thickness in the range from 6 nm to 10 nm may be provided under the silicon layer.
According to the present invention, the silicon layer is first etched halfway using a CF type gas or a mixed gas including the same. This leaves a fluorocarbon type deposition on the sides of an etched hole. Then, any residue of the silicon layer is etched using the gas of Cl2, HBr, SF6 or O2. As a result, the deposition serves as a mask to allow the silicon layer to be etched into a forwardly tapered configuration instead of being etched vertically.
The first aspect of the invention is preferable for etching only the silicon layer without etching layers under the silicon layer. However, a second aspect of the invention will be useful where etching may extend to underlying layers.
According to a second aspect of the invention, there is provided a method for etching a silicon layer comprises the steps of forming a mask pattern on a silicon layer and etching the silicon layer using a CF type gas or a mixed gas including the same.
In this method for etching, the CF type gas may be at least one kind of gas selected from among a group of gases consisting of CF4, CHF3, CH2F2 and C4F8 and a field oxide film having a thickness in the range from 0.3 xcexcm to 0.5 xcexcm may be provided under the silicon layer.
The second aspect of the invention is effective also in that the etching with a CF type gas leaves a fluorocarbon type deposition on the sides of the etched hole in the silicon layer to provide inclined etched sides similar to those obtained according to the first aspect of the invention and in that the etching gas may not be switched on the way of processing.
In the context of the present invention, the term xe2x80x9csilicon layerxe2x80x9d implies not only a silicon film formed on a substrate but also a silicon substrate. Obviously, a typical silicon layer is a polysilicon layer.